Space Shuttle Discovery’s external fuel tank does not pose an unacceptable risk to the orbiter’s planned July 1 launch, though some concerns remain, the space agency said May 31.

NASA’s work to reduce the amount of large — and potentially dangerous — pieces of foam insulation that could pop free from Discovery’s fuel tank and strike the orbiter during flight appears to have paid off, shuttle program manager Wayne Hale told reporters May 31.

“We believe we have made significant improvements since last year in the elimination of many of the hazards from foam,” Hale said during a press conference at NASA’s Kennedy Space Center spaceport in Cape Canaveral, Fl a. “There will continue to be foam coming off the external tank … what we have done in a very systematic matter is eliminated the largest hazards.”

Hale’s remarks came after he and about 100 engineers from various NASA centers across the country completed a debris verification review of Discovery’s external tank changes. A structural review of the orbiter’s fuel tank modifications, which features the removal of a 15-kilogram foam protuberance air load (PAL) ramp, is slated for the week beginning June 5.

Discovery’s STS-121 mission will mark NASA’s second shuttle flight since the 2003 loss of the Columbia orbiter and its STS-107 astronaut crew. Columbia suffered heat shield damage from errant tank foam at launch, which resulted in the orbiter’s destruction as it re-entered Earth’s atmosphere.

A foam-shedding event was observed during the first post-Columbia shuttle mission — STS-114 also aboard Discovery — but in that incident the debris did not strike the orbiter. The event, however, was a haunting reminder that foam shedding will be an ongoing challenge for NASA, and shuttle engineers already have earmarked 34 foam-covered brackets — known as ice frost ramps — on fuel tank exteriors for future work.

“We need to fly this vehicle to make sure that we made the first change right,” Hale said, stressing that the foam ramp removal is the largest structural change to the shuttle launch system since its first flight. “Then we can go and address subsequent changes.”

Foam debris concerns

External tank foam shedding has been a major concern for NASA since the Columbia accident. The need to review and address tank foam debris after the STS-114 launch last July added to the lengthy delay in the launch of the STS-121 mission.

Unlike the STS-107 and STS-114 launches, where the largest chunks of foam debris weighed 0.7 kilograms (1.6 pounds) and 0.4 kilograms respectively, Discovery’s current fuel tank is expected to shed pieces weighing less than one-tenth of a pound, Hale said.

“I don’t expect to see any one-pound pieces of foam coming off,” Hale said. But some foam likely will pop free from the ice frost ramps, and are a concern because they could strike the heat-resistant tiles along Discovery’s belly that are vital for re-entry. Ice frost ramps cover brackets that connect a tray of pressurization lines to a shuttle fuel tank’s hull.

The worst case scenario is when foam falls from an ice frost ramp with just the right size, speed and at the precise time to cause “critical damage” to an orbiter, Hale said in May .

In the past, foam pieces as large as 1.44 ounces have been seen falling from ice frost ramps, but wind tunnel and other tests by engineers suggest larger pieces weighing as much as 3.2 ounces could shake loose during flight, Hale said May 31.

“We are working our way down the list of potential debris sources to smaller and smaller releases,” Hale said.

NASA officials said the external tanks scheduled to be sued on the next two shuttle launches will not sport ice frost ramp fixes — largely because a good design is not yet available.

Whether an intermediate fix is applied to the third or fourth fuel tank scheduled to fly is still unknown. However, a final solution that might call for the installation of new titanium brackets could be in place eight shuttle flights down the line, Hale said .

Meanwhile, shuttle workers continue to prepare Discovery for flight.

The orbiter rolled out to Kennedy Space Center’s Launch pad 39B May 19, where work crews later loaded the Italian-built Leonardo supply module and other cargo for the mission into the shuttle’s payload bay.

Michael Leinbach, NASA’s launch director, said workers are currently troubleshooting an electrical glitch with Discovery’s left solid-rocket booster, though a backup system is working fine while the matter is addressed.

“I don’t really see it as an issue for us,” Leinbach said, adding that the glitch is relatively minor. “We’ve got the best in the world working on it.”